In tuning systems of the type used in television receivers utilizing voltage controlled reactive elements as frequency determining components, the frequency of operation is typically determined by a variable bias voltage provided by, for example, a potentiometer. Typically, a plurality of potentiometers are provided with each potentiometer being set to provide the proper bias voltage for a respective channel. The voltage controlled frequency determining element or elements are typcially diodes with voltage controllable capacitances such as varactor diodes. For reasons of economy it is generally desired to utilize a single diode or circuit arrangement for all of the frequencies in which received signals are expected. In the case of a VHF tuner received signals can be expected in any of channels 2-13, however, band-switching circuitry can be used to divide the operation between low-band and high-band VHF channels. In the case of a UHF tuner received signals can be expected in any of channels 14-83, and accordingly, the frequency of operation of the tuner must be variable over the entire UHF band which includes video carrier frequencies from 471.25 MHz to 885.25 MHz.
Typical tuners using varactor-type diodes require a substantial range of capacitance change of the varactor diode to cover the frequency range of interest. The capacitance characteristic of typical varactor diodes, however, is not linear with respect to the applied bias voltage. While this problem is not unduly limiting with respect to the applied tuning bias voltage, the automatic frequency control (AFC) of the tuner is deleteriously affected. The AFC circuit provides a given correction voltage for a given frequency deviation from the proper intermediate frequency. Since the varactor diode capacitance characteristic is non-linear, however a given AFC correction voltage provides a differing amount of correction depending upon the bias voltage or channel received. For example, if the proper pull-in range is provided at the low frequency channels in the band of interest, the pull-in range at the high frequency channels becomes insufficient. Conversely, if the proper or desired pull-in range is provided for the high frequency channels, the pull-in range at the low frequency channels becomes excessive which can lead to such deleterious effects as locking onto the associated sound carrier, locking onto adjacent channel carriers, and similar problems.
Prior attempts to solve the above-noted and other problems have resulted in added circuitry with attendant added complexity and cost as well as performance deficiencies. Furthermore, prior art techniques have generally lacked flexibility to accomodate diffferent tuner types with the same basic compensation circuit design.